In recent years, attendant on the spread of networks, documents hitherto distributed in the form of printed matter have come to be transmitted in the form of the so-called electronic documents. Further, books and magazines have come to be often provided in the form of the so-called electronic publishing. In order to read these pieces of information, reading from CRTs (cathode ray tubes) and liquid crystal displays of computers has conventionally been widely conducted.
However, in a light emission type display such as the CRT, it has been pointed out that the display causes conspicuous wearing on an ergonomic ground and, therefore, is unsuited to long-time reading. In addition, even a light reception type display such as a liquid crystal display is said to be similarly unsuited to reading, because of the flickering which is intrinsic of fluorescent tubes. Furthermore, both of the types have the problem that the reading place is limited to the places where computers are disposed.
In recent years, reflection type liquid crystal displays not using a backlight have put to practical use. However, the reflectance in non-display (display of white color) of the liquid crystal is 30 to 40%, which means a considerably bad visibility, as compared with the reflectance of printed matter printed on papers (the reflectance of OA papers and pocket books is 75%, and the reflectance of newspapers is 52%). In addition, the glaring due to reflectors and the like are liable to cause wearing, which also is unsuited to long-time reading.
In order to solve these problems, the media so-called paper-like displays or electronic papers have been being developed. These media mainly utilize coloration by moving colored particles between electrodes through electrophoresis or by rotating dichroic particles in an electric field. In these methods, however, the gaps between the particles absorb light, with the result that contrast is worsened and that a practical-use writing speed (within 1 sec) cannot be obtained unless the driving voltage is 100 V or higher.
As compared with the displays of these systems, electrochemical display devices for color development based on an electrochemical action (electrochromic display: ECD) are better in contrast, and they have already been put to practical use as light control glass or timepiece displays. It should be noted here that the light control glass and timepiece displays are not directly suited to the electronic paper or the like uses, since it is intrinsically unnecessary to perform matrix driving. Besides, they are generally poor in quality of black color, and the reflectance thereof remains at a low level.
In addition, in such displays as electronic papers, they are continuedly exposed to solar light or room light on a use basis. In an electrochemical display device put to practical use in the light control glass and timepiece displays, an organic material is used for forming black-colored portions. This leads to a problem concerning light resistance. In general, organic materials are poor in light resistance, and the black color density thereof is lowered through fading when used for a long time.
In order to solve these technical problems, there has been proposed an electrochemical display device using metal ions as a material for color change. In the electrochemical display device, the metal ions are preliminarily mixed into a polymer electrolyte layer, the metal is deposited and dissolved by electrochemical redox reactions, and the change in color attendant on this is utilized to perform display. Here, for example, when the polymer electrolyte layer contains a coloring material, it is possible to enhance the contrast in the case where the color change occurs.
Meanwhile, in the metal deposition type electrochemical display device based on deposition and dissolution of the metal, a threshold voltage which is the deposition overvoltage is utilized to achieve display. In each pixel, the metal is deposited when a minus voltage in excess of the threshold voltage is impressed between electrodes, whereas the metal is dissolved when a plus voltage is impressed between the electrodes.
In such a metal deposition type electrochemical display device, it is difficult to control the colored state, which is a great problem in enhancing the quality of display. For example, according to the conventional thought, the metal deposition type display device is basically designed for black-and-white display, and few attempts have been made to achieve display of gradation. This is due to the fact that it is difficult to control the colored state to an intermediate colored state with good reproducibility.
Besides, in the metal deposition type electrochemical display device, it takes a certain period of time to deposit or dissolve the metal and, therefore, problems are left as to response speed and power consumption. For example, when it is tried to once erase all pixels and to perform new writing at the time of rewriting the screen, it takes a considerable length of time and the power consumption is great.
Furthermore, in the metal deposition type electrochemical display device, there is the tendency that the display contrast would be lowered with the lapse of time, and a countermeasure against this problem is desired.
The present invention has been proposed in consideration of the above-mentioned circumstances of the related art. Accordingly, it is an object of the present invention to provide a method of driving a display apparatus by which it is possible to control the colored state of each pixel to an appropriate state, for example, a uniform state in a metal deposition type display apparatus. It is another object of the present invention to provide a method of driving a display apparatus by which it is possible to achieve display of gradations and display with high definition. It is a further object of the present invention to provide a method of driving a display apparatus by which it is possible to contrive a higher display speed and a lower power consumption of the apparatus. It is yet another object of the present invention to provide a driving method by which it is possible to sustain images for a long time in a metal deposition type display apparatus.